Solid electrolytic capacitor

ABSTRACT

A solid electrolytic capacitor comprises a capacitor element which includes an anode foil and a cathode foil rolled with a separator interposed therebetween, and a solid electrolyte layer or an electrically conductive polymer layer provided therein. The cathode foil is coated with a film of a titanium-containing compound metal nitride. The compound metal nitride is aluminum titanium nitride, chromium titanium nitride, zirconium titanium nitride or titanium carbonitride.

TECHNICAL FIELD

The present invention relates to a solid electrolytic capacitor which isproduced by rolling an anode foil and a cathode foil.

BACKGROUND ART

FIG. 2 is a sectional front view of a prior art solid electrolyticcapacitor 1, and FIG. 1 is a perspective view of a capacitor element 2(see Japanese Examined Patent Publication No. HEI 4-19695 (1992)).

The solid electrolytic capacitor 1 includes an aluminum case 3 having atop opening, the capacitor element 2 contained in the case 3, and arubber packing 30 which seals the top opening of the case 3. An upperedge portion of the case 3 is curved to fix the packing 30, and aplastic seat plate 31 is attached to the top of the case 3. Lead wires21, 21 extend from the capacitor element 2 through the packing 30 andthe seat plate 31, and then bent laterally.

As shown in FIG. 1, the capacitor element 2 includes an anode foil 4 ofan aluminum foil coated with a dielectric oxide film and a cathode foil5 of an aluminum foil, which are rolled together into a roll with aseparator 6 of a dielectric material such as paper interposedtherebetween and fixed by a tape 26. The capacitor element 2 furtherincludes a solid electrolyte such as a TCNQ(7,7,8,8-tetracyanoquinodimethane) complex salt impregnated therein, oran electrically conductive polymer layer provided therein. Lead tabs 25,25 respectively extend from the anode foil 4 and the cathode foil 5, andthe lead wires 21, 21 respectively extend from the lead tabs 25, 25.

Where the electrically conductive polymer layer is formed between thefoils 4 and 5, the capacitor element 2 is impregnated with a solutionmixture containing n-butyl alcohol as a diluent,3,4-ethylenedioxythiophene and iron p-toluenesulfonate, followed bythermal polymerization.

Although the solid electrolytic capacitor 1 having such a constructionis widely used, there is a market demand for a capacitor having asmaller size and a greater capacitance. To this end, there has beenproposed a capacitor which includes a cathode foil 5 coated with a metalnitride film as will be described below (see Japanese Unexamined PatentPublication No. 2000-114108).

An explanation will be given to the principle of the capacitanceincrease of the capacitor by coating the cathode foil 5 with the metalnitride film. In general, the dielectric oxide film is not intentionallyformed on the cathode foil 5, but formed by natural oxidation.Therefore, the capacitance C of the capacitor is equivalent to acapacitance obtained by connecting the capacitance Ca of the anode foil4 and the capacitance Cc of the cathode foil 5 in series, andrepresented by the following equation:C=Ca×Cc/(Ca+Cc)=Ca×1/(Ca/Cc+1)

That is, if the cathode foil 5 has the capacitance Cc, the capacitance Cof the capacitor is smaller than the capacitance Ca of the anode foil 4.

Where a film 52 of a metal nitride such as TiN is formed on the cathodefoil 5 by sputtering or vapor deposition, however, molecules of themetal nitride supposedly intrude into the oxide film 51 to contact analuminum base of the cathode foil 5. Therefore, the base and the metalnitride are electrically connected to each other, so that the cathodefoil 5 has no capacitance. Thus, the capacitance of the capacitor can beincreased without size increase of the capacitor.

However, this arrangement has the following drawbacks.

When the cathode foil 5 coated with the metal nitride film 52 is rolledfor production of the capacitor element 2, the film 52 is liable to beexfoliated or cracked due to a tensile force or a twist force applied tothe cathode foil 5. As a result, a leak current is increased. Further,where the electrically conductive polymer layer is formed between thefoils 4 and 5 by impregnating the capacitor element 2 with the solutionmixture containing 3,4-ethylenedioxythiophene and ironp-toluenesulfonate, the film 52 is liable to be eroded because the ironp-toluenesulfonate solution is highly acidic. This also results in theincrease in the leak current.

The cathode foil 5 coated with the metal nitride film 52 are graduallyoxidized over time. As a result, the cathode foil 5 has a capacitance,whereby the capacitance of the solid electrolytic capacitor 1 is liableto be reduced.

It is therefore an object of the present invention to provide a solidelectrolytic capacitor substantially free from the increase in leakcurrent and having a greater capacitance and a lower ESR (equivalentseries resistance).

DISCLOSURE OF THE INVENTION

An inventive solid electrolytic capacitor 1 comprises a capacitorelement 2 which includes an anode foil 4 and a cathode foil 5 rolledwith a separator 6 interposed therebetween, and a solid electrolytelayer or an electrically conductive polymer layer provided therein. Thecathode foil 5 is coated with a film of a titanium-containing compoundmetal nitride.

Alternatively, the cathode foil 5 is coated with a film comprising atitanium nitride layer. The film further comprises a titanium layerunderlying the titanium nitride layer on the cathode foil 5.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a perspective view of a prior art capacitor element;

FIG. 2 is a sectional front view of a prior art solid electrolyticcapacitor;

FIG. 3 is a schematic sectional view illustrating a part of a capacitorelement;

FIG. 4 is a sectional view for explaining how a metal nitride filmintrudes into an oxide film to reach a base; and

FIG. 5 is a perspective view illustrating a scratch test.

BEST MODE FOR CARRYING OUT THE INVENTION First Embodiment

One embodiment of the present invention will hereinafter be described indetail with reference to the attached drawings.

The solid electrolytic capacitor 1 has substantially the same overallconstruction as the prior art capacitor shown in FIG. 2. As shown inFIG. 1, a capacitor element 2 includes an anode foil 4 of an aluminumfoil having an electrochemically formed film and a cathode foil 5 of analuminum foil, which are rolled together into a roll with an insulativeseparator 6 interposed therebetween and fixed by a tape 26. Thecapacitor element 2 further includes a solid electrolyte such as a TCNQcomplex salt impregnated therein, or an electrically conductive polymerlayer provided therein. A pair of lead wires 21, 21 extend from thecapacitor element 2.

A film including a titanium (Ti) thin layer and a titanium nitride thinlayer is provided on the cathode foil 5 and, hence, has no capacitanceas described above.

The capacitor element 2 is produced in the following manner. First, analuminum foil strip cut out of an aluminum sheet is subjected to anetching process and an electrochemical process thereby to be formed witha dielectric oxide film 40 (see FIG. 3). Thus, the anode foil 4 isproduced. The etching process roughens surfaces of the anode foil 4 toincrease the surface area of the anode foil 4, thereby increasing thecapacitance of the anode foil 4.

On the other hand, titanium layers are formed on surfaces of the cathodefoil 5 by vapor deposition of titanium in vacuum, and titanium nitridelayers are formed on the titanium layers by vapor deposition of titaniumnitride in vacuum. The vapor deposition of titanium nitride is achievedby evaporating titanium in a nitrogen or ammonia atmosphere. After theformation of the titanium layers, nitrogen or the like is introducedinto a vacuum chamber for prevention of formation of an oxide film whiletitanium is evaporated. Thus, the formation of the titanium nitridelayers is achieved. Electron beam evaporation, arc plasma evaporation orthe like may be employed for the formation of the titanium layers andthe titanium nitride layers. Instead of the vapor deposition, sputteringor CVD may be employed for the film formation.

Then, the anode foil 4 and the cathode foil 5 are rolled together into aroll with an insulative separator 6 interposed therebetween, and fixedby a tape 26. Since the anode foil 4 is prepared by cutting the foilstrip from the aluminum sheet as described above, end faces of the anodefoil 4 are not formed with the dielectric oxide film. Therefore, thecapacitor element 2 is subjected to an electrochemical process to formdielectric oxide films on the end faces of the anode foil 4. Thereafter,the capacitor element 2 is thermally treated at 280° C. forstabilization of the characteristics of the dielectric oxide films.

In turn, the capacitor element 2 is impregnated with a solution mixturecontaining n-butyl alcohol as a diluent, 3,4-ethylenedioxythiophene andiron p-toluenesulfonate, followed by thermal polymerization. Thus, anelectrically conductive polymer layer 50 is formed between the foils 4and 5, whereby the capacitor element 2 is completed. The capacitorelement 2 is sealed in the case 3 in the same manner as in the priorart, whereby the solid electrolytic capacitor 1 is completed.

In this embodiment, the electrically conductive polymer layer 50 isformed of an electrically conductive polythiophene polymer, but may beformed of an electrically conductive polypyrrole or polyaniline polymer.Instead of the electrically conductive polymer layer, a solidelectrolyte layer such as of a TCNQ complex salt may be formed.

FIG. 3 is a sectional view illustrating a part of the capacitor element2 obtained after the impregnation process. The cathode foil 5 has thetitanium layers formed by vapor deposition of titanium in vacuum, andthe titanium nitride layers formed by vapor deposition of titaniumnitride in vacuum. Analysis of a section of the cathode foil 5 reveals,as shown in FIG. 3, that the thin films formed on the cathode foil 5each continuously vary from the titanium nitride layer 53 to thetitanium layer 54 toward the cathode foil 5 and no definite interface ispresent between the titanium nitride layer 53 and the titanium layer 54.This is supposedly because nitrogen of titanium nitride deposited on thetitanium layer is diffused into the titanium layer during the vapordeposition of titanium nitride or a subsequent step of the solidelectrolytic capacitor production process.

Next, the results of tests performed on solid electrolytic capacitorsactually produced according to the present invention and the prior artwill be described below.

Table 1 shows electrical characteristics of the produced solidelectrolytic capacitors in an initial state (immediately after theproduction). Prior Art Example 1 is a solid electrolytic capacitoremploying an etched aluminum foil as a cathode foil thereof, and PriorArt Example 2 is a solid electrolytic capacitor employing an aluminumfoil formed with a titanium thin film as a cathode foil thereof. PriorArt Example 3 is a solid electrolytic capacitor employing an aluminumfoil formed with a titanium nitride thin film as a cathode foil thereof.Example is the inventive solid electrolytic capacitor previouslydescribed. The solid electrolytic capacitors 1 each have a diameter of6.3 mm, a height of 6.0 mm, a rated voltage of 4V and a ratedcapacitance of 150 μF.

In Table 1, ¢Cap” indicates the capacitance of the capacitor (in unitsof IF), and “tan δ” indicates the dielectric loss of the capacitor (inunits of %). “ESR” indicates the equivalent series resistance of thecapacitor (in units of mΩ), and “LC” indicates the leak current of thecapacitor (in units of μA). Values shown in Table 1 are each calculatedas an average of measured values for 40 samples. The capacitance and thedielectric loss were measured at a frequency of 120 Hz. The equivalentseries resistance was measured at a frequency of 100 kHz. The leakcurrent was measured after a lapse of 2 minutes from application of arated DC voltage to the solid electrolytic capacitor 1. TABLE 1 CathodeCap tanδ ESR LC ΔC/C foil (μF) (%) (mΩ) (μA) (%) Prior Art Al foil 151.42.8 33.7 25 −5.2 Example 1 treated by etching Prior Art Al foil 219.92.0 33.2 19 −4.2 Example 2 with Ti deposited Prior Art Al foil 227.3 1.934.0 30 −3.4 Example 3 with TiN deposited Example Al foil 253.7 1.7 34.220 −2.1 with Ti and TiN deposited

As can be understood from Table 1, the capacitance of Example is greaterby about 10% than the capacitances of Prior Art Examples 2 and 3. Thedielectric loss of Example is slightly smaller than the dielectriclosses of Prior Art Examples. The equivalent series resistance ofExample is greater than the equivalent series resistances of Prior ArtExamples, but by a very small percentage. The leak current of Example isequivalent to the leak current of Prior Art Example 2 in which thealuminum foil formed with only the titanium thin film was employed, butis much smaller the leak currents of Prior Art Examples 1 and 3.According to the present invention, the initial capacitance of the solidelectrolytic capacitor 1 can be thus increased without deterioration ofthe electrical characteristics including the dielectric loss.

After the measurement shown in Table 1, an endurance test was performedon the capacitors of Prior Art Examples and Example. In the endurancetest, a rated voltage of 4V was applied to each of the capacitors at atemperature of 125° C. for 1000 hours. In Table 2, values of thecapacitance and the equivalent series resistance measured before andafter the test, and the change rate ΔC/C of the capacitance are shown.As in Table 1, the values are each calculated as an average of measuredvalues for 40 samples. As can be understood from Table 2, the inventivesolid electrolytic capacitor has a smaller capacitance change rate inabsolute value than the prior art solid electrolytic capacitors and,even after the endurance test, had a great capacitance. After theendurance test, the capacitors of Prior Art Examples and Example havesubstantially the same equivalent series resistance. According to thepresent invention, the change in the capacitance of the solidelectrolytic capacitor 1 over time can be reduced without deteriorationof the equivalent series resistance as compared with the prior art.TABLE 2 Initial After test Cap (μF) ESR (mΩ) Cap (μF) ΔC/C (%) ESR (mΩ)Prior Art 151.4 33.7 143.5 −5.2 35.6 Example 1 Prior Art 219.9 33.2210.7 −4.2 35.2 Example 2 Prior Art 227.3 34.0 219.6 −3.4 35.9 Example 3Example 253.7 34.2 248.4 −2.1 35.7

Second Embodiment

According to this embodiment, an aluminum foil having a film of atitanium-containing compound metal nitride such as aluminum titaniumnitride (TiAlN) or chromium titanium nitride (TiCrN) formed on surfacesthereof by an ion plating method is used as a cathode foil 5. Thealuminum foil may be preliminarily subjected or not subjected to anetching process. A capacitor element 2 produced by rolling an anode foil4 and the cathode foil 5 is impregnated with an electrically conductivepolymer and an oxidation agent in the same manner as in the firstembodiment. An alcohol solution of 40 to 60 wt % iron p-toluenesulfonateis used as the oxidation agent.

The formation of the film is achieved through deposition by the ionplating method. The ion plating may be achieved by a direct currentmethod, a high frequency method, a cluster ion beam deposition method ora hot cathode method. For the formation of the film, vacuum deposition,sputtering, thermal CVD, plasma CVD, photo CVD or laser CVD may be usedinstead of the ion plating method. The ion plating method supposedlyensures a higher adhesion strength of the film to the aluminum base thanthe sputtering method.

The applicant of the present invention produced a capacitor element 2 byusing a cathode foil 5 formed with a film of aluminum titanium nitride(TiAlN), and then produced a solid electrolytic capacitor 1 of Example 1by using this capacitor element 2.

Further, the applicant of the present invention produced a capacitorelement 2 by using a cathode foil 5 formed with a film of chromiumtitanium nitride (TiCrN), and then produced a solid electrolyticcapacitor 1 of Example 2 by using this capacitor element 2. A solidelectrolytic capacitor 1 of Prior Art Example was produced in the samemanner as in Prior Art Example 3 of the first embodiment by employing analuminum foil formed with a titanium nitride thin film as a cathode foil5.

These solid electrolytic capacitors 1 each have a rated voltage of 6.3Vand a capacitance of 180 μF, and the cases thereof each have a diameterof 6.3 mm and a height of 6.0 mm.

The capacitances (“Cap” in units of μF) of the capacitors of Examplesand Prior Art Example were measured at a frequency of 120 Hz, and theequivalent series resistances (“ESR” in units of mΩ) of the capacitorswere measured at a frequency of 100 kHz. The leak currents (“LC” inunits of μA) of the capacitors were measured after a rated directcurrent voltage was applied for two minutes. The results of themeasurement are shown in Table 3, in which the values of the electricalcharacteristics are each calculated as an average of measured values for20 samples. TABLE 3 Cap ESR LC Scratch test (μF) (mΩ) (μA) critical load(N) Prior Art 185 11.8 27 60.3 Example Example 1 220 10.7 0.7 80.3Example 2 221 10.6 0.8 82.5

In the scratch test, as shown in FIG. 5, a wedge-like diamond blade 9was dragged at a constant speed (about 2 mm/sec) while being pressedagainst the cathode foil 5 with loads applied to the diamond blade 9. Aload applied when the film was exfoliated was determined.

As can be understood from a comparison between the results describedabove, the capacitance of the solid electrolytic capacitor 1 can beincreased and the equivalent series resistance and the leak current canbe correspondingly reduced by improving the adhesion strength of thefilm to the aluminum base. Although the capacitor elements 2 of thesolid electrolytic capacitors 1 subjected to the measurement include thecathode foils 5 respectively formed with the aluminum titanium nitridefilm and the chromium titanium nitride film, a solid electrolyticcapacitor having a cathode electrode 5 formed with a zirconium titaniumnitride (TiZrN) film or a titanium carbonitride (TiCN) film supposedlyprovides substantially the same results.

In the solid electrolytic capacitor 1 having the cathode foil 5 formedwith the titanium-containing compound metal nitride film, the adhesionstrength of the film to the aluminum base of the cathode foil 5 isimproved by forming the film of the nonstoichiometric nitride compound(which cannot be represented by a simple chemical formula) on thecathode foil 5. A part of a metal in the compound metal nitride isoxidized in contact with air thereby to be passivated. Thus, a bondingforce between metal molecules in the compound metal nitride isincreased. As a result, the corrosion resistance of the film isimproved.

Therefore, the film is less liable to be exfoliated and cracked when thecathode foil 5 is rolled. Further, the film is less liable to be erodedwhen the electrically conductive polymer layer is formed. Thus, agreater capacitance and a lower ESR can be achieved without increasingthe leak current of the solid electrolytic capacitor 1.

It should be understood that the scope of the invention be not limitedby the embodiments described above. For example, the top opening of thecase 3 may be sealed with an epoxy resin. Further, the capacitor mayhave a construction of radial lead type.

INDUSTRIAL APPLICABILITY

1. In the solid electrolytic capacitor 1 having the cathode foil 5formed with the titanium-containing compound metal nitride film, theadhesion strength of the film to the aluminum base of the cathode foil 5is improved by forming the film of the nonstoichiometric nitridecompound (which cannot be represented by a simple chemical formula) onthe cathode foil 5. A part of the metal in the compound metal nitride isoxidized in contact with air thereby to be passivated. Thus, a bondingforce between metal molecules in the compound metal nitride isincreased. As a result, the corrosion resistance of the film isimproved.

Therefore, the film is less liable to be exfoliated and cracked when thecathode foil 5 is rolled. Further, the film is less liable to be erodedwhen the electrically conductive polymer layer is formed. Thus, agreater capacitance and a lower ESR can be achieved without increasingthe leak current of the solid electrolytic capacitor 1.

2. In the case of the solid electrolytic capacitor 1 having the cathodefoil 5 formed with the titanium layer and the titanium nitride layer,the change in capacitance over time can be reduced as compared with thesolid electrolytic capacitor 1 having the cathode foil 5 formed withonly the titanium layer or the titanium nitride layer. The provision ofthe titanium layer and the titanium nitride layer on the cathode foil 5increases the initial capacitance of the solid electrolytic capacitor 1as compared with the prior art solid electrolytic capacitor 1.

1. A solid electrolytic capacitor comprising a capacitor element which includes an anode foil and a cathode foil rolled with separator interposed therebetween, and a layer of a solid electrolyte or an electrically conductive polymer provided therein, wherein the cathode foil is coated with a film of a titanium-containing compound metal nitride.
 2. A solid electrolytic capacitor as set forth in claim 1, wherein the titanium-containing compound metal nitride is selected from the group consisting of aluminum titanium nitride, chromium titanium nitride, zirconium titanium nitride and titanium carbonitride.
 3. A solid electrolytic capacitor comprising a capacitor element which includes an anode foil and a cathode foil rolled with separator interposed therebetween, and a layer of a solid electrolyte or an electrically conductive polymer provided therein, the cathode foil being coated with a film comprising a titanium nitride layer, wherein the film further comprises a titanium layer underlying the titanium nitride layer on the cathode foil.
 4. A solid electrolytic capacitor as set forth in claim 1, wherein the electrolyte provided in the capacitor element is an electrically conductive polythiophene polymer.
 5. A solid electrolytic capacitor as set forth in claim 3, wherein the electrolyte provided in the capacitor element is an electrically conductive polythiophene polymer. 